This invention relates to film-forming resin compositions, a method for preparing them and their use for coating substrates, preferably metallic substrates.
GB-A-2 107 321 discloses a heat-curable water-dispersible film-forming resin composition which is the reaction product of a water-insoluble phenolic resin with an epoxy resin and a phosphoric acid.
This composition can be used alone or mixed with a water-dispersible epoxy resin for producing coating compositions. When the resin composition is mixed with a water-dispersible epoxy resin, the phenolic to total epoxy resin ratio is at least 1:5, preferably at least 1:3.
Furthermore, U.S. Pat No. 4,059,550 discloses an aqueous dispersion of a heat curable coating composition which is made from a mixture of (I) a complex polyhydroxy polyether resin which is for example terminated with epoxy groups and (II) a phenolplast resin whereby as a catalyst for the curing reaction an adduct of phosphoric acid and a glycidyl polyether of a polyhydric phenol is used.
However, formulations based on such compositions, to achieve sufficient chemical resistance for their use in food contact applications, contain an amount of phenolplast resin which renders the final coating yellow or gold in colour.
Accordingly, coating compositions disclosed in GB-A-2,107,321 and U.S. Pat. No. 4,059,550 are not suited for applications where a stable, clear or white pigmented coating system is desirable.
EP-A-0,002,718 discloses a heat curable coating composition comprising an epoxy resin and a trimellitic anhydride-glycol ester. Such a composition gives colourless coatings suitable for white pigmentation and limited use in sanitary container applications. However, formulations based on such compositions tend to be unstable with respect to viscosity versus time due to the inherent reactivity of the anhydride species and are limited in their application due to the resulting chemical resistance properties of the cured coating.
Accordingly, it remains desirable to provide a film-forming resin composition which is useful for producing clear or white pigmented coatings which are suitable for sanitary applications.